1. Field of the Invention
This invention relates to a method for the preparation of a toner material as used in a xerographic type apparatus, and more particularily, to an improved method for manufacturing toner material for use in dual component toners by a process utilizing an acid treatment of a polymeric dispersion of pigment particles.
2. Description of the Prior Art
The forming and developing of images on the surface of certain photoconductive materials by electrostatic means is now well known. Carlson, in U.S. Pat. No. 2,297,692 teaches the basic xerographic process which involves charging a photoconductive insulating layer and then exposing the layer to a light-and-shadow image which dissipates the charge on the portions of the layer which are exposed to light. The electrostatic latent image formed on the layer corresponds to the configuration of the light-and-shadow image. In another modification, a latent electrostatic image is formed on the photoconductive insulating layer by charging the layer in image configuration. A finely divided developing material comprising a colorant and called a toner is deposited on the image layer. The developing material is normally attracted to those portions of the layer which retain a charge, thereby forming a powder image corresponding to the latent electrostatic image. The powder image may then be transferred to paper or any other receiving surface. The powder image is permanently bonded to the paper by any suitable fixing means. Typically, a heating process called thermal fusing is used. For example, see the fusing processes described in U.S. Pat. Nos. 2,357,809, 2,891,011 and 3,079,342.
Over the years, many different types of toner materials have been developed for use in the various types of development methods typically employed in xerographic machines, e.g. magnetic brush development, cascade development, etc. Typically, these toners comprise various resin materials, e.g. resin materials such as styrene polymers and copolymers, that are mixed with different pigments, e.g. carbon black, to obtain a toner that is stable, have the correct triboelectric properties for proper developing in a xerographic process, have certain melting point characteristics for heat fusing, etc. Generally speaking, these toners are prepared by thoroughly mixing in a mill the resin and pigment materials (with other additives) to obtain a uniform dispersion, and then pulverizing the blended dispersion to form small particles (about 5 to 10 microns) of the material. This technique does, however, present certain disadvantages. For example, it is generally difficult to control particle size and shape within the desired range. The process itself is not an efficient one and therefore tends to increase the cost of the toner. Furthermore, and perhaps of the greater importance, is the fact that it is difficult to control the electrical properties of the final toner product which, of course, will effect copy quality to a substantial degree. If, for example, the pigment particles are not well coated with the resin, the final toner product will tend to be too conductive and will not take on a sufficient triboelectric charge. When such a toner product is used in a xerographic apparatus, either no developed image will be obtained or the image will be very light, and therefore of rather poor quality.
Various attempts have been made to overcome some of the above described problems. For example, U.S. Pat. No. 4,071,670 describes a process for the formation of polymeric particles (for subsequent use in electrostatic powder coating) of a size variation within the range of 5 to 25 microns by a suspension polymerization process. Specifically, the particles are formed by mixing a monomer in an aqueous solution of water and a stabilization agent, subjecting the mixture to high shear agitation, polymerizing the monomer and then recovering the polymeric particles. U.S. Pat. No. 4,077,804 describes a method of producing toner particles by an in-situ polymerization process. Chemical Abstracts, Physical and Analytical Chemistry Sections, Vol. 88, No. 11, Abstract No. 161466P describes toners prepared by a process involving direct polymerization of a pigmented monomer. The inhibition of polymerization of the monomer by the pigment is eliminated by coating the pigment with a reactive silane. However, not all of these procedures are successful in eliminating the disadvantages described hereinabove, and in some cases, the processes described introduce other problems. For example, in some instances the yield of toner product is rather poor. In addition, there is sometimes a tendency for the toner particles to agglomerate.
It is also known to prepare toners by polymerization techniques where large amounts of surfactants are needed in order to provide a stable dispersion. Typically, large organic molecule type surfactants are combined with either strong acid salts, such as phosphates, sulfates, or electron donating type groups such as polyvinyl alcohol and amine. The strong acid salts tend to largely increase the electrical conductivity of the toner particles, while the electron donating groups, such as polyvinyl alcohol, tend to promote unfavorable triboelectric charge properties.